The present invention relates to an alkaline storage battery using zinc as a negative electrode such as, for example, nickel-zinc, silver-zinc or zinc-manganese storage battery and particularly to an improvement of the zinc electrode used as a negative electrode.
An alkaline secondary battery using zinc as a negative electrode has a high energy density and a good output characteristic and can be produced inexpensively, so is expected as a power source for portable devices and electric vehicles.
However, since the solubility of zinc as an active material of the negative electrode is high, there arise problems. For example, zincate ions which have been dissolved out during electric discharging form dendrites and cause a short circuit, or the electrode itself undergoes a change in shape, resulting in lowering of the utilization factor. Due to these problems, an alkaline secondary battery using zinc as a negative electrode and having a sufficiently long cycle life has not been obtained yet.
For solving the above problems, various improvements have been made with respect to electrolyte, separator and electrode. However, there has not been obtained yet an alkaline secondary battery using zinc as a negative electrode and having a sufficiently long cycle life.
Particularly, the separator is generally known as means for suppressing dendrite physically, and various studies have been made using polyolefin non-woven fabric and other materials. The separator is required to have ion conductivity, gas permeability and capability of preventing a short circuit caused by dendrites. To meet these requirements, improvements have been made; for example, a metal is held in the separator (Japanese Patent Laid Open Nos. JP57-197758A and JP59-63672A, a copolymer is used as the separator material (Japanese Patent Publication No. JP55-24660B and Patent Laid Open No. JP60-253152A), or the separator is treated with a surface active agent (Japanese Patent Laid Open No. JP4-141951A). However, these methods involve a complicated process and therefore a method capable of affording a satisfactory effect through a simpler process has been desired.
Moreover, zincate ions which were dissolved out into an electrolyte layer during electric discharge are segregated onto electrodes at the time of charging, resulting in a shape change of the electrodes and decrease in the charge and discharge capacity of the battery. It has been impossible to prevent such inconvenience even by improvement of the separator.
Further, applying a polyvinyl alcohol directly to electrodes to improve the cycle characteristic in a bound state to the separator has also been tried, as shown in Japanese Patent Laid Open No. 1457/1991. However, the cycle characteristic is not satisfactory in practical use because the polyvinyl alcohol used as a binder is not considered satisfactory in its ability to prevent the growth of dendrite.
It is the object of the present invention to overcome the above-mentioned problems, particularly to provide a storage battery using zinc as a negative electrode in which the cycle characteristic is improved by suppressing dendrite and shape change in an initial stage thereof.